This is an open-label, PK study in multiple myeloma subjects who are currently receiving thalidomide-containing therapy or are newly diagnosed. The study will consist of a screening phase, a baseline phase, a PK phase with a 5-day period of thalidomide treatment (200 mg/day), and an end-of-study evaluation on Day 6. Subjects will have frequent blood samples drawn for PK assessments on Days 5 and 6 in an inpatient setting.